Generally, such protective tubes are made of metal. They are used to protect optical fibers in optical cables, in particular under-sea cables, or metal wires which may be associated with one or more optical fibers, in particular in live cables or guard cables in overhead power transmission lines. The components protected in this way are embedded in a sealing material filling the tube.
Such tubes are extruded or are manufactured by longitudinally welding a strip shaped into a tube, the strip itself being corrugated or smooth.
Protective tubes for optical fibers may be tubes of small diameter, i.e. about 2.5 mm, and of thickness about 0.1 mm to 0.3 mm. They are then referred to as "microtubes". Generally, the optical fibers are inserted into the microtube while it is being manufactured, usually by its longitudinal edges being welded together by means of a laser torch, for example. The fibers are further embedded in the sealing material filling the microtube. The microtube is usually filled while the fibers are being enclosed therein.
In particular, Document U.S. Pat. No. 4,852,790 describes an installation for making a microtube around a fiber or a bundle of fibers, and for simultaneously filling said microtube with a sealing material as it is being made.
In that installation, a tube for injecting sealing material opens out into a microtube which is still slightly open and which is receiving the fiber(s). Said tube opens out downstream from the point at which the edges of the microtube are welded together. The fibers are pushed into place by an inert gas blown into a fiber guide tube.
Such protective tubes or microtubes are not always quite fluid-tight, and when they are obtained by welding, they sometimes have small pores at the welded edges.
Known means for detecting faults along very long metal tubes or wires use ultrasonic methods, or eddy currents. However, such methods remain ineffective when the products to be monitored are below a certain size, and therefore they cannot be applied to detecting faults in protective microtubes for optical fibers. Other techniques for detecting faults in tubes consist in injecting a test gas, in particular helium, into the tube to be monitored, and in detecting any traces of helium in the surrounding atmosphere. Those techniques are not suitable for very long tubes which are small in cross-sectional area, when such tubes are already filled with a sealing material which prevents the test gas from propagating under pressure along the tubes.
An object of the present invention is to provide sealing-fault detection for detecting sealing faults in protective tubes for electrical and/or optical cables, in which the protected components are already embedded in a gel-type sealing material which fills the tube, which detection is effective regardless of the size of the tubes and is therefore applicable both to protective tubes or microtubes for optical fibers and to protective tubes which are much larger in cross-sectional area, and which protect electrical cables or electrical and optical components.